The present invention relates to an improved rotating cathode magnetron suitable for sputtering or reactive sputtering of materials from a tubular cathode target onto a stationary or moving substrate as well as a method of operating the magnetron. The magnet assembly of the inventive magnetron is arranged in such a way that local variations in the plasma race-track are generated which may provide novel advantages to the sputtering process. In addition, the novel magnet assembly is particularly suitable for curvilinear arrangements.
In standard non-reactive metallic sputter mode, sputtering with planar magnetrons is known. The most important inconvenience is the formation of a groove of erosion in the target material, whereby this groove, and the plasma generating it, are often referred to as a xe2x80x9crace trackxe2x80x9d. The non-uniform erosion profile is inherently associated with the magnet configuration below the target. As a consequence, the target has to be replaced just before the erosion groove depth at any point equals the target thickness. Typically only 30% of the target material is consumed before the target has to be changed which makes it a very costly process because of labour costs, down time as well as the expense of target materials.
During reactive sputtering (i.e. the plasma contains one or more gases that will react with the target material) using planar magnetrons, additional problems of arcing and plasma instability are encountered. Both of these problems have been overcome by the introduction of cylindrical rotating target magnetrons. Firstly, with rotating target magnetrons no race track erosion profile (corresponding to the magnet configuration) is formed and the material consumption on the target can be up to 80%. Secondly, due to the nature of the rotating cathode, less problems and more stable processes are encountered during reactive sputter deposition. Nevertheless, large amounts of material are deposited on shields which are physically located between the target and the substrate to inhibit the deposition of target material on those locations where it is not desired. Therefore, regular cleaning and extensive precautions (e.g. water cooling) have to be foreseen on these shields to reduce the risk of flaking. Flakes of material from shields can contaminate the sputtered surface.
Coating of large substrates in a uniform way during a single passage (i.e. typical condition in glass and web coating), is one of the most critical processes. Control may be obtained by placing additional wedge shaped shields (introducing another source of contaminating particle generation) or by changing the strength of the magnetic field lines (using magnets with different magnetisation or at different distances from the target). The latter solution may introduce non-uniform wear and/or consumption of the target material.
Cylindrical magnetrons have some other disadvantages which are typical for their geometry. The magnets are mounted on a static bar which is located within the rotating cylindrical target tube. The width of the magnet configuration is kept small which means that the turns at the end are quite sharp. Known magnet assemblies do not allow optimal configuration of the magnets in an end turn which results in reduced plasma confinement and increased electron loss at both ends of the target. It is desirable to have the magnets as close as possible to the target tube in order to produce the highest magnetic field strength at the surface of the cathode. In addition, at both ends of the target tube, where the magnets (and the race track) form a U-bend, sore target material is removed. The top of the xe2x80x9cUxe2x80x9dxe2x80x94bend presents a length of the plasma race-track which remains at the same longitudinal position as the target rotates. This leaves a circular groove round the target tubes at both ends. Eventually, target life is limited by the depth of this groove as it is highly undesirable to deposit the underlying material of the tube onto the substrate.
Rotating cathode sputtering magnetrons with a stationary internal magnet assembly are known, e.g. from U.S. Pat. Nos. 4,422,916, 5,364,518 or WO 96/21750. In particular U.S. Pat. No. 5,364,518 and WO 96/21750 propose magnet assemblies which produce an elongate plasma xe2x80x9crace-trackxe2x80x9d above the target which has a shape comprising a spaced apart pair of parallel straight lengths terminated at each end by end portions or xe2x80x9cUxe2x80x9d turns. U.S. Pat. No. 5,364,518 proposes controlling target erosion in the end portions by means of widening the track of the race-track in the end positions. As explained in WO 96/21750, the method according to U.S. Pat. No. 5,364,518 has the disadvantage that the wider track of the race-track in the end portions may result in instability of the plasma due to the reduced field strength and resulting electron loss caused by the wider spacing of the magnets. Instead, WO 97/21750 proposes to make the end portions of the race-track xe2x80x9cpointedxe2x80x9d, i.e. to elongate the end portions into an acute angle, e.g. triangular or to make them semi-elliptical or parabolic in form. The disadvantage of making the end portions pointed, in particular triangular in shape is that the radius at the point is very small. This results in a high loss of electrons as they attempt to navigate this tight bend. To achieve reduce electron loss it may be considered to increase the magnetic field in this position in order to bind the electrons more closely to the track. However, increasing the magnetic field increases the plasma density and hence, the target erosion. Further, although WO 97/21750 proposes sophisticated geometrical shapes for the end portions of the race-track, e.g. parabolic or semi-elliptic, the only disclosed method of producing such refined track geometries is the use of discrete sections of magnets, the so-called xe2x80x9clumpedxe2x80x9d magnet method. It is not possible to accurately tailor the race-track to a sophisticated geometric form such as a parabola by means of lumped magnetsxe2x80x94the steps between the magnets generate a castellated appearance which bears little relationship to a smooth curve (see FIG. 3 in the following).
U.S. Pat. No. 5,645,699 describes the use of anodes to influence the deposition rate onto the substrate during reactive sputtering. This known method starts from the assumption that there is inevitable loss of electrons in the turns at the end of the race-track.
The present invention has the object of providing a sputtering magnetron and a method of operating the same which provides improved control over sputtering performance.
A further object of the present invention is to provide a sputtering magnetron and a method of operating the same which provides improved uniformity of erosion at the ends of the target.
Still a further object of the present invention is to provide a sputtering magnetron and a method of operating the same which provides improved uniformity of deposition onto the substrate.
Another object of the present invention is to provide a sputtering magnetron and a method of operating the same which provides a plasma race-track with reduced loss of electrons in the end portions thereof.
Yet a further object of the present invention is to provide a sputtering magnetron and a method of operating the same which provides improved target utilisation while allowing novel and useful ways of altering the coating sputtered onto the substrate.
The present invention may provide a sputtering magnetron with a rotating cylindrical target and a stationary magnet assembly, said magnet assembly being adapted to produce an elongate plasma race-track on the surface of said target, said elongate race-track having substantially parallel tracks over a substantial portion of its length and being closed at each end by end portions, wherein the spacing between the tracks of said race-track is increased locally to materially effect sputtering onto a substrate.
The present invention also includes a method of operating a sputtering magnetron with a rotating cylindrical target and a stationary magnet assembly, comprising the steps of: generating an elongate plasma race-track on the surface of said target using said magnet assembly, said elongate race-track having substantially parallel tracks over a substantial portion of its length and being closed at each end by end portions, and increasing the spacing between the tracks of said race-track locally to materially effect sputtering onto a substrate.
The present invention may provide the advantage that the requirement for shields can be substantially reduced. As a consequence, the cost and time consuming maintenance of these shields can be lowered, while their detrimental effect on process and product quality can be minimised. This property can be achieved by reducing the unwanted deposition of material in the region between the target and the substrate. Though excess material is still brought into the vacuum system, it can be gathered at non-critical locations, e.g. on shields between the target and the chamber walls. These shields have no direct relation with the substrate and so require less precautions, less maintenance and have no effect on the process or film quality.
In addition, shields may no longer be needed to control the film thickness uniformity over (large) substrates. In addition, the current technology of changing magnet strengths and distances, resulting in non-uniform consumption of the target material can be overcome. Precise control of sputter efficiency towards the substrate for any desired position on the substrate can be obtained with the present invention, while maintaining uniform erosion of the cylindrical target tube. This means that standard tubes can be used for every possible uniformity of erosion profile.
Furthermore, the present invention allows freedom in the configuration of the magnet assembly. The U-turns at the end of the target tube can be defined freely, allowing better control of the magnetic field. Therefore, plasma configurations in the turns and on straight zones can be achieved with which the loss of electrons is reduced. The radius of the U-turns can be varied even to values larger than the diameter of the target tube. In addition, in accordance with the present invention, the direction of manetic field adjacent the magnet assembly may be arranged perpendicular to the target surface, allowing the creation of the largest possible magnetic field strength on the target surface. Likewise, the top surface of the magnets in the magnet assembly may be arranged parallel to the target tube which enables the closest possible positioning with respect to the target, giving the largest possible magnetic efficiency.
The circular erosion groove at the end of the target tube (due to the U-turns) known from conventional devices can be reduced and in some cases even be eliminated. A spoon or elliptical (i.e. more than semi-elliptical) shape is preferred for the race-track in the end zones in accordance with one embodiment of the present invention. Even old and worn targets, for which the groove is so deep that the underlying material becomes visible, can be used again without the risk of depositing the wrong material on the substrate.
In accordance with the present invention simultaneous metal and reactive sputtering can be achieved when the race-track is arranged to traverse the back side of the cathode.
The present invention also includes a sputtering magnetron having a magnet assembly and a target, said magnet assembly being adapted to produce a curvilinear plasma race-track on the surface of said target, said magnet assembly including: a first section for generating a magnetic field associated with a first magnetic polarity; a second section being spaced from said first section and generating a magnetic field associated with a second magnetic polarity, said first and second sections defining a magnetic field suitable for enclosing said curvilinear race-track; wherein one of the first and second sections includes at least one magnet and the other of said first and second sections is terminated by a soft magnetic material forming a magnetic circuit with said magnet. By the termination of the second section is meant that the second section defines the magnetic pole which is the interface between magnetic material and non-magnetic material, i.e. the atmosphere above the magnet array and the target.
The dependent claims define separate embodiments of the invention. The present invention, its embodiments and advantages will now be described with reference to the following drawings.